Field of the Disclosure
Embodiments of the invention relate to a display device including an oxide semiconductor thin film transistor (TFT) and a method for manufacturing the same.
Discussion of the Related Art
An electronic display device includes a thin film transistor (TFT) at each pixel so as to turn on a data voltage applied to the pixel or drive the pixel. An amorphous silicon TFT, polycrystalline silicon TFT, an oxide semiconductor TFT, etc, are known types of TFTs.
The oxide semiconductor TFT has mobility greater than the amorphous silicon TFT and can be manufactured in a low temperature process. Further, the oxide semiconductor TFT has an advantage of transparency and is capable of transmitting visible light. Thus, the oxide semiconductor TFT is suitable for a high resolution display device or a transparent display device.
One problem is that an oxide semiconductor material of the TFT dissolves easily in an acid solution. Thus, when a metal stacked on the oxide semiconductor material is wet etched, the oxide semiconductor may be damaged or degraded by the etchant. To avoid this, the metal stacked on the oxide semiconductor may be dry etched using plasma, for example. However, damage to the surface of the oxide semiconductor may be caused by the plasma used in the dry etching.
Another alternative to protect the oxide semiconductor is to form an etch stopper on the oxide semiconductor to prevent back etching of the oxide semiconductor layer of the TFT. However, a photomask process for forming the etch stopper has to be added during manufacture of the device. A photomask process is a photolithographic technology process for sequentially performing a series of steps including a thin film deposition process, a photoresist application process, a photomask alignment process, an exposure process, a development process, an etching process, a strip process, etc., to pattern a thin film. A reduction in the number of photomask processes results in a reduction in the manufacturing cost and an increase in device yield.
Also, a gate insulator may be formed of silicon oxide (SiOx) to prevent changes in characteristics of the oxide semiconductor TFT. However, it takes more time to perform a wet etching process and a dry etching process of silicon oxide (SiOx) than silicon nitride (SiNx), which is widely used as a gate insulator material. Therefore, a method for simultaneously etching the etch stopper and the gate insulator, each of which is formed of silicon oxide (SiOx), through the dry etching process may be considered. However, because it is highly likely that a photoresist pattern is degraded due to a long processing time, it is difficult to execute this method. Thus, a separate photomask process for patterning the gate insulator formed of silicon oxide (SiOx) is added to the device fabrication.
In a display driving circuit, a common voltage Vcom may be supplied to the pixels through a metal layer. In this instance, the metal has to be exposed so it can be contacted by etching the gate insulator covering the metal. When the gate insulator is formed of silicon oxide (SiOx), the gate insulator is etched through a separate photomask process.
A passivation layer covering the oxide semiconductor TFT may use an organic passivation material. The organic passivation layer may be formed of photoacryl. Also, a pixel electrode may be connected to a source of the oxide semiconductor TFT by etching a portion of the passivation layer at a position outside the oxide semiconductor TFT. However, this method has to form a hole to expose the pixel electrode to the organic passivation layer in an opening inside the pixel. The pixel opening being a transparent portion of the pixel that does not block light. Pixel portions that would block light include wiring and the TFT. Therefore, an additional process tolerance margin for forming the hole in the opening is necessary. The additional space needed for the margin leads to a reduction in an aperture ratio of the pixel.